[emscripten] CI is broken on osx

[sergz]

Although the commit (#6220#comment:11) was landed yesterday and had a chance to run successfully once the things changed and now CI is broken. In particular the command brew install python3 fails with the message

Error: python 2.7.14 is already installed
To upgrade to 3.6.4_3, run `brew upgrade python`

Additional observations while trying to fix it

On the other hand, when one does what is recommended, python2 is replaced and there is no python2, but almost all our scripts require python2. So, in order to have it I am calling brew install python2; after upgrading, so physically there is python2 now but it's not found as python. FYI, if one calls brew link python2 then it prints

Warning: python@2 is keg-only and must be linked with --force
Note that doing so can interfere with building software.

If you need to have this software first in your PATH instead consider running:
  echo 'export PATH="/usr/local/opt/python@2/bin:$PATH"' >> ~/.bash_profile

And at that path there is no python, so scripts don't work either.

One expected it to happen one day anyway, so IMO we should encourage migration to python3 more and start to use virtualenv for python2.

However it does not matter how it's called, e.g. either as virtualenv -p python2 third_party/python2 (PATH is correct) or even as virtualenv -p /usr/local/opt/python@2/bin/python2 third_party/python2, it says /usr/local/bin/virtualenv: /usr/local/opt/python/bin/python2.7: bad interpreter: No such file or directory and the remedy is to call brew link --force python2 before.

[abpbot]

A commit referencing this issue has landed:
​Issue 6439 - fix emscripten CI for osx